A conventional valve system for an intake and exhaust valve mechanism of an engine has a roller provided at a location opposed to a cam to smooth sliding contact between the valve system and the cam.
The structure of this roller is disclosed, for example, in Japanese Patent Publication No. 22085/93 or Japanese Laid-Open Utility Model Publication No. 32403/89. This structure has a bushing type bearing interposed between a roller and a roller shaft, or has a needle bearing interposed between a roller and a roller shaft. In either structure, the occurrence of wear due to the friction of the roller with a rocker arm body inhibits the smooth action of the valve system, and shortens the life of the valve system.
The structure disclosed in Japanese Patent Publication No. 22085/93 has an oil groove provided on the circumference of the outer periphery of the roller shaft for holding a lubricating oil, and an oil hole provided in the bushing to smooth the supply of the lubricating oil to the inner peripheral surface and outer peripheral surface of the bushing, thereby enhancing a bearing function. The structure disclosed in Japanese Laid-Open Utility Model Publication No. 32403/89 has a solid lubricant film coated on the recessed inner surface of a rocker arm corresponding to both side surfaces of the needle roller, thereby reducing wear of a partial slide surface between the roller and the rocker arm body.
The structure disclosed in Japanese Laid-Open Utility Model Publication No. 32403/89, however, poses considerable difficulty in doing work for coating the solid lubricant film only on the inner peripheral surface of the narrow recess of the rocker arm body. This work makes the manufacturing process tiresome and raises the cost of the product. Furthermore, lubrication between the needle bearing and each of the roller and roller shaft depends on the oil supplied from the end of the needle bearing. This results in poor lubricating performance at the initial stage of engine operation.
The structure disclosed in Japanese Patent Publication No. 22085/93, on the other hand, has the oil groove provided in the roller shaft, and the oil hole in the bushing, in order to maintain satisfactory lubrication of the inner and outer peripheral surfaces of the bush. This makes the manufacturing process for the roller shaft and bushing tiresome, and raises the cost of the product. In addition, burrs occurring during formation of the oil groove and the oil hole damage the slide surfaces of the bushing, roller, and roller shaft. The provision of the oil groove in the bushing also decreases the area of sliding contact of the bushing with the roller and the roller shaft, thus increasing the load per unit area of the bushing slide surface that is generated at the push of the roller by the cam. This leads to decreased durability of the bushing.
Usually, lubrication for the entire valve system is performed by the penetration of lubricating oil splashes flying from around the valve system. Particularly at the initial stage of operation, such as at the start of the engine, the temperature is low and the viscosity of the lubricating oil is high, so that penetration of splashes is minimal. With a valve system of the rocker arm type, moreover, it takes time for the lubricating oil to ascend to the cylinderhead. Partly for this reason, lubrication of the roller and the bushing may be insufficient. Even with the structure disclosed in Japanese Patent Publication No. 22085/93, formation of a lubricating film is difficult at the start of the engine, and at the initial stage of operation, in particular. This may cause a marked damage to the valve system, such as scoring of the surface of sliding contact between the bushing and each of the roller shaft and the roller.
Besides, the area of sliding contact between the bushing and each of the roller shaft and the roller is larger than in a valve system using a needle bearing. Thus, friction increases, but the area of sliding contact is nearly uniform in the direction of the roller width, so that the lubricating oil film becomes minimally discontinuous. With a valve system using a needle bearing, on the other hand, the area of sliding contact between the needle bearing and each of the roller shaft and the roller is smaller than in a valve system using a bushing. Thus, friction in the entire needle bearing is small, but the needle bearing may be inclined relative to the central axis of the roller. As a result, friction occurs unevenly in the width direction of the needle bearing. The lubricating oil film becomes nonuniform in the direction of the roller width, making the lubricating film apt to become discontinuous.
This problem may be pronounced particularly in the case of an engine, such as a diesel engine, in which a lubricating oil may become easily contaminated with impurities (e.g., soot). In this type of engine, the use of a valve system with a needle bearing inserted therein can make a lubricating film formed on the peripheral surface of the needle roller discontinuous even more easily, because of the small area of sliding contact of the needle bearing and impurities involved in the lubricating oil. Thus, the wear of the valve system may be aggravated. Even when a solid lubricant film is formed on the entire needle bearing, friction occurs nonuniformly. Thus, the solid lubricant film can be damaged easily, and wear of the valve system is difficult to reduce sufficiently.
The present invention has been accomplished in the light of the above circumstances. Its object is to provide a valve system of an engine which is adapted to act smoothly for a long term, without causing loss of the lubricating properties of the slide surfaces of the bushing, even in the state of undersupply of the lubricating oil that occurs at the initial stage of operation of the engine.